Laminated Composite Report

INTRODUCTION TO LAMINATED COMPOSITES
In materials science, a composite laminate is an assembly of layers of fibrous composite materials which can be joined to provide required engineering properties, including in-plane stiffness, bending stiffness, strength, and coefficient of thermal expansion. The individual layers consist of high-modulus, high-strength fibers in a polymeric, metallic, or ceramic matrix material. Typical fibers used include cellulose, graphite, glass, boron, and silicon carbide, and some matrix materials are epoxies, polyimides, aluminium, titanium, and alumina.

Layers of different materials may be used, resulting in a hybrid laminate. The individual layers generally are orthotropic (that is, with principal properties in orthogonal directions) or transversely isotropic (with isotropic properties in the transverse plane) with the laminate then exhibiting anisotropic (with variable direction of principal properties), orthotropic, or quasi-isotropic properties. Quasi-isotropic laminates exhibit isotropic (that is, independent of direction) inplane response but are not restricted to isotropic out-of-plane (bending) response. A laminated composite material consists of several layers, each comprised of a matrix and fibres. The composite laminate design starts with the selection of the number of layers and their orientation. Once the number of layers and layer orientation are selected, a Laminate Stacking Sequence (LSS) is chosen. An LSS is considered heterogeneous when there is preferential stacking of specific layer orientation in different locations through the thickness of the laminate (Schmit & Farshi 1977). Thick laminates with heterogeneous LSS are created by clumping layers of similar orientation. An LSS is said to be
homogeneous when layer orientations are evenly distributed through the laminate thickness. The ability to generate a homogeneous LSS depends on the number of layers, their orientation and position. Due to production of composite materials in many combinations and forms, each layer may have similar or dissimilar material properties with different fibre orientations under varying stacking sequences.

Generally, the structural properties of the laminated composites such as stiffness, strength, and dimensional stability have all been found to depend on the LSS. In view of each property having different relations with a particular stacking sequence, the choice of the stacking sequence suited for a particular application may entail a compromise. It is essential to know the dynamic characteristics of such structures subjected to dynamic loads in 27 complex environmental conditions. The structural components made of composite materials such as aircraft wings, helicopter blades, vehicle axles, turbine blades and machine tool structures can be approximated as laminated composite beams (Kapuria & Alam 2006).

1.2. LAMINATION
Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance or other properties from the use of differing materials. A laminate is a permanently assembled object by heat, pressure, welding, or adhesives. There are different lamination processes, depending on the type of materials to be laminated. The materials used in laminates can be the same or different, depending on the processes and the object to be laminated. An example of the type of laminate using different materials would be the application of a layer of plastic film—the "laminate"—on either side of a sheet of glass—the laminated subject. Vehicle windshields are commonly made by laminating a tough plastic film between two layers of glass. This is to prevent shards of glass detaching from the windshield in case it breaks. Plywood is a common example of a laminate using the same material in each layer. Glued and laminated dimensioned timber is used in the construction industry to make wooden beams, Glulam, with sizes larger and stronger than can be obtained from single pieces of wood. Another reason to laminate wooden strips into beams is quality control, as with this method each and every strip can be inspected before it becomes part of a highly stressed component as shown in figure (1.1). Electrical equipment such as transformers and motors usually use steel laminations to form the core of coils used to produce magnetic fields. The thin laminations reduce the loss due to eddy currents.

1.3. CARBON FIBRE REINFORCED PLASTIC (CRPF)
Laminated composites have excellent in-plane mechanical properties, however their through-thickness strength and toughness is less impressive as is evident in their tendency to delaminate (Bond et al. 2011). Reliability is closely related to delamination toughness which in turn is related to material strength under tension, compression and impact conditions (Sohn & Hu 1998). In aircraft composites, approximately 70% of structural failures have been found to initiate from the joints (Abdul Razak & Othman 2011). The use of composites has therefore been limited in safety critical applications, most notably in aircraft, where any kind of failure mid-flight is likely catastrophic and fatal (Bond et al 2011). The solutions to technical and economic challenges that would allow composite materials to achieve maximum weight saving potential are beyond the
current state of the art (Aero Index Ltd 2011).

Carbon fibre reinforced plastic (CFRP), more commonly referred to simply as carbon fibre, is a composite material produced by impregnating carbon fibre fabric with a polymer resin. Epoxy resins are also excellent
adhesives, and are therefore commonly used both to cure the fibre matrix as well as bond multiple composite parts together; this is called co-curing (Cognard 2006). Carbon fibre has achieved widespread acceptance as a lightweight alternative to materials such as steel and aluminium as it provides much greater strength and stiffness for the same mass of material as well as having good chemical resistance (Sohn & Hu 1998, Sohn et al. 2000). It is however more expensive and is therefore generally reserved for applications where the greater
cost is acceptable in exchange for an increase in performance and efficiency. Whilst it exhibits very high specific strength and stiffness, carbon fibre is a brittle material that, like all laminated composites, is susceptible to damage caused by various loadings. These include static loading, low energy impact and environmental factors such as moisture and extreme temperatures which can have a considerable effect on mechanical performance, fatigue behavior and the nature of failure in composite joints (Ashcroft et al. 2000, Aero Index Ltd 2011, Sohn et al. 2000). Low energy impact in particular can cause sub-surface damage that may not be visible on the laminate surface (Sohn et al. 2000). Bond et al. (2011) suggests that a 10J low velocity impact on a composite panel can reduce its compressive strength by up to 35%.